With no guarantee of it actually working, this UFO-O2 Detector on Amazon is supposedly “designed to sense these disturbances and will signal their presence by flashing an LED and beeping.” Here’s one of the reviews from user Mild Mild West: “This device is only good for detecting the greys, which are not the ones to be looking out for. This would be a lot more useful if it also detected Reptoids.” Product page. One more review from user Dmitri Ulinov:
This piece of equipment has saved my butt (if you know what I mean) on more than one occasion. However, you always need to check that it is properly functioning because if The Greys find you with one of these in your truck on the day that it happens not to be working, you will pay dearly on the exam table. I kid you not!